The present invention relates to a center differential which is mounted on a four-wheel drive vehicle for differentiating the front and rear wheels and, more particularly, to a differential equipped with a mechanism for restraining the differential.
In a vehicle such as a passenger car, the four-wheel drive state, in which the driving force is transmitted to all the front and rear wheels, is superior in running stability and ability to withstand rough road driving, as is well known in the art. In the four-wheel drive state in which the front and rear wheels are connected directly to the output shaft of a transmission, moreover, it is also well known in the art that a braking phenomenon is caused during cornering due to the difference in the number of rotations of the front and rear wheels. Therefore, the so-called "full-time four-wheel drive car" (i.e., "full-time 4 WD") is equipped with a center differential to differentiate the front and rear wheels. With this center differential, however, in case either the front or rear wheels stick, no driving force will be transmitted to the remaining wheels. In order to avoid this trouble, a differential restraining means is provided in addition to the center differential. An example of this provision is disclosed in Japanese Patent KOKAI No. 55-72420.
In the differential restraining means disclosed, there is used a differential gear set which is composed of a pair of side gears, a pinion gear meshing with the side gears, and a shaft and casing holding the pinion gear. The output shaft of a transmission is so connected to the shaft of the pinion as to rotate the pinion gear therearound, and one of the side gears is connected to a rear wheel output shaft whereas the other side gear is connected to a front wheel output shaft, thus constituting the center differential. A multi-disc clutch is interposed between the pinion holding the casing and the other side gear and is engaged to restrain the differential by a piston which is hydraulically driven.
In recent trials, on the other hand, a viscous coupling has been used in the transfer to perform not only the differentiation of the front and rear wheels but also the control of the transmission torque due to the difference in the number of rotations.
In the differential restraint for engaging the differential restraining clutch by a hydraulically actuated piston, not only a hydraulic circuit but also electric devices such as sensors or arithmetic elements for controlling hydraulic devices are required which complicate control devices so that the system becomes seriously expensive. Since, moreover, an oil pressure source is naturally required, it can be utilized as it is if the transmission is automatic, but a special oil pressure source has to be provided in the case of a manual transmission. Therefore, another problem is that the production cost is raised because the parts of the automatic transmission cannot be commonly used.
If a differential restraining piston to be hydraulically driven is used, it is conceivable to adjust the transmission torque capacity of the differential restraining clutch to change the torque distribution between the front and rear wheels in accordance with the running state by adjusting the oil pressure for driving the piston. If this control is accomplished, the running performance can be better improved. For this control, however, there are required a number of sensors of high precision and quick response. In fact, however, an electric system or a hydraulic system may be delayed, which would cause a problem in that proper differential restraint cannot be achieved.
The aforementioned differential gear set having the paired side gears and the pinion gear may have disadvantages due to their large external size or weight. In order to set the torque distribution between the front and rear wheels at a ratio other than 50:50, moreover, a special structure is required to change the constructions of the gears drastically, thus raising still another problem of a further increase in the weight.
Therefore, the differential gear set having the paired side gears and the pinion meshing with the former is currently replaced by a planetary gear set in the center differential. In case the planetary gear set is used in the center differential, it is the current practice to use helical gears so as to reduce the noise (such as tooth hitting noises due to back-lash). Special parts for receiving the thrust are required to raise a further problem of rising production cost.
In the aforementioned viscous coupling, on the other hand, a number of discs are arranged in a viscous fluid such as silicone oil to transmit the torque through the shearing resistance of the viscous fluid following relative rotations of the discs. As a result, the transfer using the viscous coupling will transmit the higher torque as the difference between the number of rotations of the front and rear wheels increases. Thus, such transfer is liable to be encountered by a braking phenomenon during cornering having a large difference in the number of rotations between the front and rear wheels and is troubled by deterioration in fuel mileage. Moreover, the viscous coupling per se is expensive and is remarkably difficult to tune for attaining the desired characteristics.
The so-called "TORSEN differential" using a worm and a worm wheel is also known in the art. This differential can perform the differential restraint of torque response but cannot avoid the braking phenomenon during cornering from a theoretical standpoint. Furthermore, the torque distribution cannot be set at a ratio other than 50:50. The structure is also complex and expensive. In addition, a further disadvantage is that another planetary gear set is required for setting the torque distribution at a ratio other than 50:50.